Apparatuses for finely milling solid particles in a material to be treated (mill base) by the use of milling media (beads) and dispersing or milling the particles in a liquid, have been known. As such apparatuses, bead mills (wet-type media-dispersing apparatus, media mill, etc.) using beads on the order of millimeter (mm) size as milling media, have been widely used. In apparatuses used for practical production, the material to be treated, which is typically a slurry of solid particles in a liquid, is stirred together with beads, and the solid particles are milled by the shearing action of the beads and dispersed in the liquid to obtain particles finely divided to submicron size.
However, with bead mills using beads of mm size, it is difficult to obtain extremely fine nano particles with a particle size of at most 100 nanometer, which have been demanded in recent years. The reason is that it is difficult to apply uniform shearing forces to nano particles. Accordingly, bead mills using beads of a few tens μm have been recently developed on a laboratory scale to make it possible to apply more uniform shearing forces to nano particles, whereby it becomes possible to conduct milling and dispersion to nearly obtain a primary particle size of nanometer size in the laboratory.
On the other hand, ultrasonic dispersing apparatuses are known that are used for dispersion in a liquid/liquid system for, e.g., production of emulsion, or milling and dispersion of solid particles in a solid/liquid system. By this method, it is possible to impart a more uniform collision of the particles with one another, whereby at some low concentration range of particles or in a laboratory setting, it is possible to conduct milling and dispersion to a level to nearly obtain a primary particle size of nanometer size. In the ultrasonic dispersing apparatuses, collision of particles with one another can be promoted by shock waves generated during decay of cavitation, whereby it becomes possible to conduct milling and dispersion.
JP-A-2000-351916 describes a pigment-dispersing apparatus applicable at production level, combining the above-mentioned bead mill using beads of mm size and ultrasonic irradiation. More specifically, a media mill and a vessel are connected by a pipe, and a mixture of a liquid medium and solid pigments to be uniformly dispersed is circulated therethrough. The pressure in the vessel is reduced during the dispersing treatment, and an ultrasonic-generating mechanism is disposed at a part of the vessel. However, in the ultrasonic-generating mechanism used in such an apparatus or method, irradiation of the mixture with ultrasonic waves is conducted for the purpose of preventing sedimentation of a dispersion having the contents dispersed with the media mill, and not for obtaining nano particles uniformly milled and dispersed.
As mentioned above, with a bead mill using very fine beads of a few tens μm size, it becomes possible to conduct milling and dispersion to nearly a primary particle size of nanometer size in a laboratory setting, but there is a problem in that it is difficult to separate nano particles and beads of a few tens μm size dispersed in a liquid, and such apparatus cannot be used at practical production level. Further, when beads of mm size, for example, beads of 0.5 mm (500 μm), are used, it has been found that although separation of beads is possible, it is impossible to conduct milling and dispersion to nearly a primary particle size of nanometer size.
On the other hand, a method has been known in that milling and dispersion to the primary size level can be made by only irradiation with ultrasonic waves. However, in such a method, the conditions for sufficiently generating the collision of particles with one another are extremely narrow, and the range in which the propagated wave energy can be exerted is extremely narrow, whereby only a small amount of material can be treated. Accordingly, from the aspect of cost performance, such a method is hardly used for practical production.
Further, a method combining irradiation with ultrasonic waves and a bead mill has been proposed as mentioned in the above JP-A-2000-351916, but the irradiation with ultrasonic waves is conducted in a tank of a large capacity at practical production level, and no milling media (balls or beads) are contained in the tank, and therefore the extent of collision of particles with one another by irradiation with ultrasonic waves is insufficient to obtain finely milled solid particles. In order to mill solid particles and disperse them in a liquid by only the effect of irradiation with ultrasonic waves, there is no other way than making the tank extremely small, and such a method cannot be used for practical production due to low efficiency.
Various methods have been attempted as mentioned above, but there is still a problem that in the application for the purpose of milling and dispersing nano particles, such methods have not reached practical production level using beads of mm size.